I’ve been asked to respond to the question “To what extent are man-made CO2 emissions contributing to climate change?”

The short answer is:  ‘we don’t know.’ The reason is that we don’t know how to disentangle natural internal variability from the effects of CO2–driven warming

Even the IPCC doesn’t claim to know exactly. The most recent IPCC assessment report says it is ‘extremely likely’ to be  ‘more than half.’ ‘More than half’ is not very precise.

Given the IPCC’s neglect of multi-decadal and longer time scales of natural internal variability, I regard the extreme confidence of their conclusion to be unjustified.

So here is my personal assessment, using the jargon of the IPCC:  Man-made CO2emissions are as likely as not to contribute less than 50% of the recent warming.